1. Field of the Invention
This invention relates to flip chip bonding tool tips and, more particularly, to dissipative ceramic flip chip bonding tips for bonding electrical connections.
2. Description of the Prior Art
Integrated circuits have different methods of attachment to a lead frame. One method is an ultrasonic wire bond, both ball and wedge, where individual leads are connected to individual bond pads on the integrated circuit with wire. Wire bonding uses “face-up” chips with a wire connection to each pad. Bump or “flip chip” microelectronic assembly is the direct electrical connection of face-down—“flipped”—electronic components onto substrates, circuit boards, or carriers by means of conductive bumps on the chip bond pads.
Flip chip components are predominantly semiconductor devices. Components such as passive filters, detector arrays, and MEMs devices are also used in flip chip form. Flip chip is sometimes referred to as Direct Chip Attach (DCA) as the chip is attached directly to the substrate, board, or carrier by the conductive bumps. Automotive electronics, electronic watches, and a growing percentage of cellular phones, pagers, and high speed microprocessors are assembled with flip chips.
The bump serves several functions in the flip chip assembly. Electrically, the bump provides the conductive path from chip to substrate. The bump also provides a thermally conductive path to carry heat from the chip to the substrate. In addition, the bump provides part of the mechanical mounting of the die to the substrate. The bump also provides a spacer, preventing electrical contact between the chip and substrate conductors and acting as a short lead to relieve mechanical, strain between board and substrate.
Gold stud bumps are placed on die bond pads through a modification of the “ball bonding” process used in conventional wire bonding. In ball bonding, a tip of a gold bond wire is melted to form a sphere. A wire bonding tool presses this sphere against an aluminum bond pad, applying mechanical force, heat, and ultrasonic energy to create a metallic connection. The wire bonding tool next extends the gold wire to a connection pad on the board, substrate, or lead frame and makes a “stitch” bond to the pad, finishing by breaking off the bond wire to begin another cycle.
For gold stud bumping, the first ball bond is made as described but the wire is then broken close above the ball. The resulting gold ball, or “stud bump” remaining on the bond pad provides a permanent and reliable connection through the aluminum oxide to the underlying metal. After placing the stud bumps on a chip, the stud bumps may be flattened—“coined”—by mechanical pressure to provide a flatter top surface and more uniform bump heights while pressing any remaining wire tail into the ball. Each bump may be coined by a tool immediately after forming, or all bumps on the die may be simultaneously coined by pressure against a flat surface in a separate operation following bumping.
Bonding tool tips must be sufficiently hard to prevent deformation under pressure and mechanically durable so that many bonds can be made before replacement. Typical flip chip bonding tips available on the market today are made of an insulator of zirconia tuffened alumina (Al2O3)—aluminum oxide—tungsten carbide, or titanium carbide. These insulators are very hard compounds that have been successfully used on commercial machines as these compounds provide a reasonably long life in use as a flip chip bonding tool.
The problem, however, is that an electrostatic discharge from the bonding tool or transient currents from the machine can damage the very circuit the tool is bonding. Flip chip bonding tool tips must be electrically designed to produce a reliable electrical contact, yet prevent electrostatic discharge damage to the part being bonded. Certain prior art devices have a one-or-more volt emission when the tip makes bonding contact. This could present a problem as a one-volt static discharge could generate a 20 milliamp current to flow, which, in certain instances, could cause the integrated circuit to fail due to this unwanted current.